The success and profitability of many industries depends directly on the ability to operate equipment reliably and on demand. Equipment down time necessitated by repair or replacement of components adds unacceptably to the costs of operation, and reduces the ability to generate income from operation of that piece of equipment. It becomes critical, therefore, for all components used in such equipment to be rugged, reliable, and cost effective. It also becomes critical for such components to provide controlled operation that performs its function without adversely affecting the equipment of which it is a component.
In many industries, such as mining, power generation, oil and gas, and marine, heavy duty engines drive such equipment. Such engines may have displacements that may range from 5L (305 c.i.d.) to 300L (18,300 c.i.d.) or more. The first step in the operation of such equipment depends on the ability to reliability start the equipment's engine under a wide variety of conditions and environments. In industries that require the use of heavy duty engines, engine air starters that operate from an air/gas supply are typically used to perform such engine starting.
Such engine air starters typically employ a turbine air motor driven by the air/gas supply to rotate a shaft that is coupled to an engine starter drive. The engine starter drive is the mechanism that meshes with the ring gear and actually starts the engine. One such engine starter drive is known as an inertia drive. An inertia drive is coupled to the air motor output shaft via clutch plates, and includes a screw shaft on which a pinion gear rides. To start the engine, the turbine air motor is driven from the source of air/gas, which drives its output shaft. This rotary motion is coupled through the clutch plates to drive the screw shaft. The inertia of the pinion gear causes it to be translated along the screw shaft and into engagement with a ring gear of the engine. Once the pinion gear reaches the end of its travel along the screw shaft, it is fully meshed with the engine's ring gear. Continued rotation of the screw shaft rotates the pinion gear, which in turn rotates the ring gear of the engine to start the engine. Once the engine starts, it begins to accelerate the ring rear faster than the rotation of the screw shaft. This results in the pinion gear being translated along the screw shaft away from and out of engagement with the ring gear.
As can well be imagined by those skilled in the art, once the pinion gear has reached the end of its travel along the screw shaft and is fully meshed with the ring gear of the static engine, there is developed a large torque as the pinion gear attempts to accelerate the ring gear of the engine. As this torque is transmitted through the screw shaft, the clutch plates will slip if the torque rises above the spring force holding the clutch plates together. As the ring gear begins to rotate, the slip becomes less until the ring rear is being rotated by pinion gear without any slip at all.
The holding force on the clutch plates is critical to proper operation of the engine starter drive. If the clutch plates do not slip at the appropriate torque, either the engine will not start or serious damage may occur to either the engine or the starter, including the shearing of shafts, the breaking of gear teeth, etc. That is, if the force on the clutch plates is too light, the starting torque of the engine may not be overcome and the clutch plates will simply continue to slip without starting the engine. If the force on the clutch plates is too high, mechanical failure of engine or starter components may result (shearing shafts, breaking gear teeth, etc.). Such results are unacceptable. Further, with the cost sensitive nature of industry, both the engine and the starter are designed to operate within a fairly narrow tolerance band of torques before failure will occur.
In a conventional inertia drive engine starter, such as that shown in the partial cross sectional illustration of FIG. 6, the force that holds the clutch plates together is provided primarily by six pressure springs 100. These six pressure springs 100 are distributed around the periphery of the shaft head 102 on which the clutch disks 104 are mounted. The clutch body 106 is secured axially to the screw shaft 108 by a head screw/backstop 110. A meshing spring 112 also provides a force on the clutch plates 104 through the screw shaft 108 and clutch body 106. As is recognized by those skilled in the art, the meshing spring 112 is provided to allow some recoil of the screw shaft 108 and pinion 114 should the pinion 114 strike the engine ring gear (not shown) in its attempt to mesh therewith. The typical force applied by this meshing spring may be approximately 50 pounds, while the force applied by the six pressure springs 100 is typically approximately 500 pounds.
With the conventional construction as illustrated in FIG. 6, the inertia drive engine starter has a load schematic as illustrated in FIG. 7. As may be seen from this load schematic illustration, both the pressure springs and the meshing spring 112 apply their force against the clutch plate stack 104. These two combined spring forces from the pressure springs 100 and the meshing spring 112 act to compress the clutch plate stack 104 against the frame 116 to prevent slip between the clutch plates 104. These forces may be better understood with reference to the free body diagram of FIG. 8. As may be seen from this free body diagram, the pressure spring force 118 and the meshing spring force 120 on the clutch stack 104 is countered by the frame reaction force 122.
Unfortunately, with both the pressure springs and the meshing spring applying their forces against the clutch plates 104 in this configuration, any variation in the meshing spring force 120 will directly affect the ability of the clutch plates to maintain torque transfer without slippage. That is, in this conventional configuration, variations in the force of the meshing spring, which is meant to serve primarily a shock absorbing function, now directly affects the torque transmission capability of the entire clutch stack 104 in its primary function of transmitting torque to start the engine. As a result, the level of torque transmitted by the clutch plates is not controlled to a narrow range, but instead is subject to wide variations that may adversely affect starting performance as discussed above. In an exemplary embodiment of the conventional inertia drive engine starter having 500 pound pressure spring force and 50 pound meshing spring force, slip will occur in a range anywhere between 300 to 330 pounds. This wide, uncontrolled range of torque at which the clutch plates will slip increases the cost of ownership of such a drive resulting from increased wear if the slip occurs at too low a torque value, and excessive stress on the engine and starting components when the torque level is too high.
There exists, therefore, a need in the art for torque transmission level control within an inertia engine starter drive to ensure proper starting without damaging the engine or the starter drive components. Further, there exists a need for such a system to be cost effective.